1. Field of the Invention
The invention pertains to a bandpass filter with pseudo-elliptic response of waveguide type. Such a filter is used in particular in high-frequency transmission systems.
2. Description of the Prior Art
The mass-market development of broadband bidirectional transmission devices requires the use of a filtering device exhibiting considerable constraints in terms of selectivity, bandwidth, bulkiness and cost. These constraints are very considerable at the level of the filtering carried out on the antenna side to isolate transmission and reception where signals lying in two very close bands have to be isolated from one another.
Among the filtering technologies usable for millimeter frequencies, the technologies of waveguide type exhibit a quality factor high enough to meet the requirements. The waveguide filters most commonly used are nowadays E-plane filters with dielectric insert and H-plane filters with inductive irises.
Beyond 40 GHz, and for highly selective filters, it is preferable to use H-plane filters with inductive irises. FIG. 1 represents a bandpass filter of order 3 with four inductive irises possessing a Chebyshev type response. Such a filter, in order to be highly selective, has to have a high order N, giving rise to an increase in the number of irises which is equal to N+1. However, the increase in the number of irises causes an increase in the size of the filter.
In order to increase the selectivity of an iris filter, it is known, for example, from the article by W. MENZEL et al, “Planar integrated waveguide diplexer for low cost millimeter-wave applications” EUMC, pp 676-680, September 1997, to introduce transmission zeros near the passband. The introduction of transmission zeros produces a quasi-elliptic response which improves the selectivity of the filter. On the other hand, the introduction of transmission zeros is achieved by adding sections of guide (or resonant cavities) placed perpendicularly to the principal axis of the filter, therefore rendering the filter less compact. Furthermore, the number and the frequency positioning of the transmission zeros is limited on account of the method of implementation.
An aim of the invention is to propose an H-plane filter with inductive irises which exhibits a quasi-elliptic response while retaining the same compactness as a filter having a Chebyshev response. A second aim is to be able to use a large number of transmission zeros. For this purpose, there is proposed a waveguide filter with inductive iris in which at least one floating insert is placed in an iris.